The present invention relates to data transmission systems in which the start bits of messages being transmitted by stations within such systems have a characteristic which allows them to be easily distinguished from data bits in the messages and, more particularly, to such systems in which a start bit has at least a portion of a time duration different than a data bit so that a start bit can be easily distinguished from a data bit.
Computer technologies involving data processing and data transmission have been adapted for use in building control and supervision because they offer advantages and economies which building control systems had not theretofore achieved. The prior art building control and supervision systems for maintaining comfort control and for monitoring fire and security points within the building were not able to easily minimize energy consumption in maintaining desired comfort levels nor minimize installation and hardware costs in wiring the central control function to the various loads being monitored and controlled. Instead of wiring the control function to each load separately and instead of requiring excessive amounts of hardware to achieve energy management functions, the modern building control system uses computer technology for connecting the various loads of a building over a single channel to the central control function and uses software and/or digital technology for achieving energy management control.
In a system comprising a central control station connection over a single communication channel to a plurality of remote stations, each station has a unique address and communicates with other stations by transmitting digital messages each having an address portion specifying the particular station to which the message is directed to receive and process the message. The message will also have other portions, such as control and data portions, which may be utilized to require the addressed station to perform certain functions such as providing a status report, answer a polling message with any alarm messages which it may have to transmit to the central controller, requiring a remote station to perform a control function such as turning loads on or off, as well as requiring a wide variety of other functions.
In a typical data transmission system of this nature, any message transmission is begun with a start bit or a start sequence to inform the receiving stations that a message transmission is beginning. Prior art systems constructed start bits in much the same fashion as they constructed data bits, i.e. from one or more bits which could not be easily distinguished from data bits. As a result, data processing systems on occasion generated errors through confusing data bits with start bits. Thus, it was possible for such a system to interpret a start bit as a data bit resulting in the processing operation at the receiving station becoming unsynchronized to the data bits in the message which it was processing at the time. Likewise, it was possible for such a station to interpret a data bit as a start bit which again resulted in lack of synchronization between the specific bits making up the data message and the interpretation of those bits by the receiving station.